Field of the Invention
This invention relates generally to an RF transmitter module for a cellular radio that includes a delta-sigma modulator and, more particularly, to an RF transmitter module for a cellular radio that includes a delta-sigma modulator having a plurality of interleaving dynamic element matching (DEM) circuits providing interleaved digital bits at a reduced clock rate.
Discussion of the Related Art
Traditional cellular telephones employ different modes and bands of operations that have been supported in hardware by having multiple disparate radio front-end and baseband processing chips integrated into one platform, such as tri-band or quad-band user handsets supporting GSM, GPRS, etc. Known cellular receivers have integrated some of the antenna and baseband data paths, but nevertheless the current state of the art for mass mobile and vehicular radio deployment remains a multiple static channelizing approach. Such a static architecture is critically dependent on narrow-band filters, duplexers and standard-specific down-conversion to intermediate-frequency (IF) stages. The main disadvantage of this static, channelized approach is its inflexibility with regards to the changing standards and modes of operation. As the cellular communications industry has evolved from 2G to 3G, 4G and beyond, each new waveform and mode has required a redesign of the RF front-end of the receiver as well as expanding the baseband chip set capability, thus necessitating a new handset. For automotive applications, this inflexibility to support emerging uses is prohibitively expensive and a nuisance to the end-user.
Providing reliable automotive wireless access is challenging from an automobile manufacturers point of view because cellular connectivity methods and architectures vary across the globe. Further, the standards and technologies are ever changing and typically have an evolution cycle that is several times faster than the average service life of a vehicle. More particularly, current RF front-end architectures for vehicle radios are designed for specific RF frequency bands. Dedicated hardware tuned at the proper frequency needs to be installed on the radio platform for the particular frequency band that the radio is intended to operate at. Thus, if cellular providers change their particular frequency band, the particular vehicle that the previous band was tuned for, which may have a life of 15 to 20 years, may not operate efficiently at the new band. Thus, this requires automobile manufactures to maintain a myriad of radio platforms, components and suppliers to support each deployed standard, and to provide a path to upgradability as the cellular landscape changes, which is an expensive and complex proposition.
Known software-defined radio architectures have typically focused on seamless baseband operations to support multiple waveforms and have assumed similar down-conversion-to-baseband specifications. Similarly, for the transmitter side, parallel power amplifier chains for different frequency bands have typically been used for supporting different waveform standards. Thus, crucially, receiver front-end architectures have typically been straight forward direct sampling or one-stage mixing methods with modest performance specifications. In particular, no prior application has required a greater than 110 dB dynamic range with associated IP3 factor and power handling requirements precisely because such performance needs have not been realizable with complementary metal oxide semiconductor (CMOS) analog technology. It has not been obvious how to achieve these metrics using existing architectures for CMOS devices, thus the dynamic range, sensitivity and multi-mode interleaving for both the multi-bit analog-to-digital converter (ADC) and the digital-to-analog converter (DAC) is a substantially more difficult problem.
Software-defined radio architectures do not exist in the automotive domain, but have been proposed and pursued in other non-automotive applications, such as military radios with multi-band waveforms. However, in those arenas, because of vastly different waveform needs, conflicting operational security needs and complex interoperability requirements, a zero-IF approach has proven technically difficult. Known software defined radios have typically focused on backend processing, specifically providing seamless baseband operations to support multiple waveforms. The modest performance specifications haven't demanded anything more aggressive from front-end architectures. Straight-forward direct sampling or 1-stage mixing methods have been sufficient in the receiver. For software defined radios that employ delta-sigma modulators, the component function is commonly found after a down conversion stage and has low-pass characteristics. With regard to the transmitter, parallel multiple power amplifier chains to support differing frequency bands and waveform standards have been sufficient for meeting the requirements.